[Nexus 7 (2013) Q&A] k23m:
Jtag + uart

I was checking the JTAG interface and noticed that this test header also has an active 1.8V UART.

The connections are:

1 = Rx

2 = Tx

3 = VCC 1.8V
​

This UART initially runs 115200bps, 8N1, and outputs very useful diagnostic messages. For example, if your Nexus 7 is stone-dead because of faulty battery, the UART is quite alive (on USB power) for 8.8 seconds and reporting emergency_shutdown_by_lowlow_battery:

Android Bootloader - UART_DM Initialized!!!
[0] welcome to lk

[10] platform_init()
[10] PcbValue for Project ID= 1
[10] target_init()
[80] eMMC_Total_Sector = 0x3a40000 (61079552)
[80] mmc: taac: 15000000, mclk_rate: 49152000, nsac: 100
[90] mmc: Read timeout set: 7472800 clk
[90] hw reset: 1
[90] QUP IN:bl:8, ff:32, OUT:bl:8, ff:32
[100] WR:Wrote 0x22c01aa to out_ff:0x12460110
[100] RD:Wrote 0x40101ab to out_ff:0x12460114
[100] idx:8, rem:1, num:2, mode:0
[110] QUP_IN_NOT_EMPTY:0x10; 0x20
[110] QUP_IN_NOT_EMPTY delay retry : 0
[160] idx:0, rem:1, num:2, mode:0
[170] QUP_IN_NOT_EMPTY:0x10; 0x20
[170] QUP_IN_NOT_EMPTY delay retry : 1
[220] idx:0, rem:1, num:2, mode:0
[220] QUP_IN_NOT_EMPTY:0x10; 0x20
[230] QUP_IN_NOT_EMPTY delay retry : 2
[280] idx:0, rem:1, num:2, mode:0
[280] QUP_IN_NOT_EMPTY:0x10; 0x20
[290] QUP_IN_NOT_EMPTY delay retry : 3
[340] idx:0, rem:1, num:2, mode:0
[340] QUP_IN_NOT_EMPTY:0x10; 0x20
[350] QUP_IN_NOT_EMPTY delay retry : 4
[400] idx:0, rem:1, num:2, mode:0
[400] QUP_IN_NOT_EMPTY:0x10; 0x20
[410] QUP_IN_NOT_EMPTY delay retry : 5
[460] Error time : 6
[480] WR:Wrote 0x22c01aa to out_ff:0x12460110
[480] RD:Wrote 0x40101ab to out_ff:0x12460114
[490] idx:8, rem:1, num:2, mode:0
[490] QUP_IN_NOT_EMPTY:0x10; 0x20
[490] QUP_IN_NOT_EMPTY delay retry : 0
[550] idx:0, rem:1, num:2, mode:0
[550] QUP_IN_NOT_EMPTY:0x10; 0x20
[550] QUP_IN_NOT_EMPTY delay retry : 1
[610] idx:0, rem:1, num:2, mode:0
[610] QUP_IN_NOT_EMPTY:0x10; 0x20
[610] QUP_IN_NOT_EMPTY delay retry : 2
[670] idx:0, rem:1, num:2, mode:0
[670] QUP_IN_NOT_EMPTY:0x10; 0x20
[670] QUP_IN_NOT_EMPTY delay retry : 3
[720] idx:0, rem:1, num:2, mode:0
[730] QUP_IN_NOT_EMPTY:0x10; 0x20
[730] QUP_IN_NOT_EMPTY delay retry : 4
[780] idx:0, rem:1, num:2, mode:0
[790] QUP_IN_NOT_EMPTY:0x10; 0x20
[790] QUP_IN_NOT_EMPTY delay retry : 5
[840] Error time : 6
[860] WR:Wrote 0x22c01aa to out_ff:0x12460110
[870] RD:Wrote 0x40101ab to out_ff:0x12460114
[870] idx:8, rem:1, num:2, mode:0
[870] QUP_IN_NOT_EMPTY:0x10; 0x20
[880] QUP_IN_NOT_EMPTY delay retry : 0
[930] idx:0, rem:1, num:2, mode:0
[930] QUP_IN_NOT_EMPTY:0x10; 0x20
[940] QUP_IN_NOT_EMPTY delay retry : 1
[990] idx:0, rem:1, num:2, mode:0
[990] QUP_IN_NOT_EMPTY:0x10; 0x20
[990] QUP_IN_NOT_EMPTY delay retry : 2
[1050] idx:0, rem:1, num:2, mode:0
[1050] QUP_IN_NOT_EMPTY:0x10; 0x20
[1050] QUP_IN_NOT_EMPTY delay retry : 3
[1110] idx:0, rem:1, num:2, mode:0
[1110] QUP_IN_NOT_EMPTY:0x10; 0x20
[1110] QUP_IN_NOT_EMPTY delay retry : 4
[1170] idx:0, rem:1, num:2, mode:0
[1170] QUP_IN_NOT_EMPTY:0x10; 0x20
[1170] QUP_IN_NOT_EMPTY delay retry : 5
[1230] Error time : 6
[1250] [i2c_read][addr=55][reg=2c] retry 3 times failed
[1250] asus: [get_battery_capacity] battery capacity read fail !
[1280] WR:Wrote 0x22c01aa to out_ff:0x12460110
[1280] RD:Wrote 0x40101ab to out_ff:0x12460114
[1280] idx:8, rem:1, num:2, mode:0
[1290] QUP_IN_NOT_EMPTY:0x10; 0x20
[1290] QUP_IN_NOT_EMPTY delay retry : 0
[1350] idx:0, rem:1, num:2, mode:0
[1350] QUP_IN_NOT_EMPTY:0x10; 0x20
[1350] QUP_IN_NOT_EMPTY delay retry : 1
[1400] idx:0, rem:1, num:2, mode:0
[1410] QUP_IN_NOT_EMPTY:0x10; 0x20
[1410] QUP_IN_NOT_EMPTY delay retry : 2
[1460] idx:0, rem:1, num:2, mode:0
[1470] QUP_IN_NOT_EMPTY:0x10; 0x20
[1470] QUP_IN_NOT_EMPTY delay retry : 3
[1520] idx:0, rem:1, num:2, mode:0
[1530] QUP_IN_NOT_EMPTY:0x10; 0x20
[1530] QUP_IN_NOT_EMPTY delay retry : 4
[1580] idx:0, rem:1, num:2, mode:0
[1580] QUP_IN_NOT_EMPTY:0x10; 0x20
[1590] QUP_IN_NOT_EMPTY delay retry : 5
[1640] Error time : 6
[1660] WR:Wrote 0x22c01aa to out_ff:0x12460110
[1670] RD:Wrote 0x40101ab to out_ff:0x12460114
[1670] idx:8, rem:1, num:2, mode:0
[1670] QUP_IN_NOT_EMPTY:0x10; 0x20
[1680] QUP_IN_NOT_EMPTY delay retry : 0
[1730] idx:0, rem:1, num:2, mode:0
[1730] QUP_IN_NOT_EMPTY:0x10; 0x20
[1740] QUP_IN_NOT_EMPTY delay retry : 1
[1790] idx:0, rem:1, num:2, mode:0
[1790] QUP_IN_NOT_EMPTY:0x10; 0x20
[1790] QUP_IN_NOT_EMPTY delay retry : 2
[1850] idx:0, rem:1, num:2, mode:0
[1850] QUP_IN_NOT_EMPTY:0x10; 0x20
[1850] QUP_IN_NOT_EMPTY delay retry : 3
[1910] idx:0, rem:1, num:2, mode:0
[1910] QUP_IN_NOT_EMPTY:0x10; 0x20
[1910] QUP_IN_NOT_EMPTY delay retry : 4
[1970] idx:0, rem:1, num:2, mode:0
[1970] QUP_IN_NOT_EMPTY:0x10; 0x20
[1970] QUP_IN_NOT_EMPTY delay retry : 5
[2030] Error time : 6
[2050] WR:Wrote 0x22c01aa to out_ff:0x12460110
[2050] RD:Wrote 0x40101ab to out_ff:0x12460114
[2060] idx:8, rem:1, num:2, mode:0
[2060] QUP_IN_NOT_EMPTY:0x10; 0x20
[2060] QUP_IN_NOT_EMPTY delay retry : 0
[2120] idx:0, rem:1, num:2, mode:0
[2120] QUP_IN_NOT_EMPTY:0x10; 0x20
[2120] QUP_IN_NOT_EMPTY delay retry : 1
[2170] idx:0, rem:1, num:2, mode:0
[2180] QUP_IN_NOT_EMPTY:0x10; 0x20
[2180] QUP_IN_NOT_EMPTY delay retry : 2
[2230] idx:0, rem:1, num:2, mode:0
[2240] QUP_IN_NOT_EMPTY:0x10; 0x20
[2240] QUP_IN_NOT_EMPTY delay retry : 3
[2290] idx:0, rem:1, num:2, mode:0
[2300] QUP_IN_NOT_EMPTY:0x10; 0x20
[2300] QUP_IN_NOT_EMPTY delay retry : 4
[2350] idx:0, rem:1, num:2, mode:0
[2350] QUP_IN_NOT_EMPTY:0x10; 0x20
[2360] QUP_IN_NOT_EMPTY delay retry : 5
[2410] Error time : 6
[2430] [i2c_read][addr=55][reg=2c] retry 3 times failed
[2440] asus: [get_battery_capacity] battery capacity read fail !
[2460] WR:Wrote 0x22c01aa to out_ff:0x12460110
[2470] RD:Wrote 0x40101ab to out_ff:0x12460114
[2470] idx:8, rem:1, num:2, mode:0
[2470] QUP_IN_NOT_EMPTY:0x10; 0x20
[2480] QUP_IN_NOT_EMPTY delay retry : 0
[2530] idx:0, rem:1, num:2, mode:0
[2530] QUP_IN_NOT_EMPTY:0x10; 0x20
[2540] QUP_IN_NOT_EMPTY delay retry : 1
[2590] idx:0, rem:1, num:2, mode:0
[2590] QUP_IN_NOT_EMPTY:0x10; 0x20
[2600] QUP_IN_NOT_EMPTY delay retry : 2
[2650] idx:0, rem:1, num:2, mode:0
[2650] QUP_IN_NOT_EMPTY:0x10; 0x20
[2650] QUP_IN_NOT_EMPTY delay retry : 3
[2710] idx:0, rem:1, num:2, mode:0
[2710] QUP_IN_NOT_EMPTY:0x10; 0x20
[2710] QUP_IN_NOT_EMPTY delay retry : 4
[2770] idx:0, rem:1, num:2, mode:0
[2770] QUP_IN_NOT_EMPTY:0x10; 0x20
[2770] QUP_IN_NOT_EMPTY delay retry : 5
[2830] Error time : 6
[2850] WR:Wrote 0x22c01aa to out_ff:0x12460110
[2850] RD:Wrote 0x40101ab to out_ff:0x12460114
[2860] idx:8, rem:1, num:2, mode:0
[2860] QUP_IN_NOT_EMPTY:0x10; 0x20
[2860] QUP_IN_NOT_EMPTY delay retry : 0
[2920] idx:0, rem:1, num:2, mode:0
[2920] QUP_IN_NOT_EMPTY:0x10; 0x20
[2920] QUP_IN_NOT_EMPTY delay retry : 1
[2980] idx:0, rem:1, num:2, mode:0
[2980] QUP_IN_NOT_EMPTY:0x10; 0x20
[2980] QUP_IN_NOT_EMPTY delay retry : 2
[3030] idx:0, rem:1, num:2, mode:0
[3040] QUP_IN_NOT_EMPTY:0x10; 0x20
[3040] QUP_IN_NOT_EMPTY delay retry : 3
[3090] idx:0, rem:1, num:2, mode:0
[3100] QUP_IN_NOT_EMPTY:0x10; 0x20
[3100] QUP_IN_NOT_EMPTY delay retry : 4
[3150] idx:0, rem:1, num:2, mode:0
[3160] QUP_IN_NOT_EMPTY:0x10; 0x20
[3160] QUP_IN_NOT_EMPTY delay retry : 5
[3210] Error time : 6
[3230] WR:Wrote 0x22c01aa to out_ff:0x12460110
[3240] RD:Wrote 0x40101ab to out_ff:0x12460114
[3240] idx:8, rem:1, num:2, mode:0
[3240] QUP_IN_NOT_EMPTY:0x10; 0x20
[3250] QUP_IN_NOT_EMPTY delay retry : 0
[3300] idx:0, rem:1, num:2, mode:0
[3300] QUP_IN_NOT_EMPTY:0x10; 0x20
[3310] QUP_IN_NOT_EMPTY delay retry : 1
[3360] idx:0, rem:1, num:2, mode:0
[3360] QUP_IN_NOT_EMPTY:0x10; 0x20
[3370] QUP_IN_NOT_EMPTY delay retry : 2
[3420] idx:0, rem:1, num:2, mode:0
[3420] QUP_IN_NOT_EMPTY:0x10; 0x20
[3430] QUP_IN_NOT_EMPTY delay retry : 3
[3480] idx:0, rem:1, num:2, mode:0
[3480] QUP_IN_NOT_EMPTY:0x10; 0x20
[3480] QUP_IN_NOT_EMPTY delay retry : 4
[3540] idx:0, rem:1, num:2, mode:0
[3540] QUP_IN_NOT_EMPTY:0x10; 0x20
[3540] QUP_IN_NOT_EMPTY delay retry : 5
[3600] Error time : 6
[3620] [i2c_read][addr=55][reg=2c] retry 3 times failed
[3620] asus: [get_battery_capacity] battery capacity read fail !
[3650] emergency_shutdown_by_lowlow_battery: low low battery voltage detect
[3650] WR:Wrote 0x23001d4 to out_ff:0x12460110
[3660] RD:Wrote 0x40101d5 to out_ff:0x12460114
[3660] idx:8, rem:1, num:2, mode:0
[3670] WR:Wrote 0x23001d4 to out_ff:0x12460110
[3670] WR:Wrote 0x380 to out_ff:0x12460114
[3670] idx:6, rem:1, num:1, mode:0
[3680] WR:Wrote 0x20301d4 to out_ff:0x12460110
[3680] RD:Wrote 0x40101d5 to out_ff:0x12460114
[3690] idx:8, rem:1, num:2, mode:0
[3690] asus: [smb345_vflt_setting] Set float voltage, retval=e8 setting=e8
[3700] WR:Wrote 0x20301d4 to out_ff:0x12460110
[3700] WR:Wrote 0x3e8 to out_ff:0x12460114
[3700] idx:6, rem:1, num:1, mode:0
[3710] WR:Wrote 0x23001d4 to out_ff:0x12460110
[3710] RD:Wrote 0x40101d5 to out_ff:0x12460114
[3710] idx:8, rem:1, num:2, mode:0
[3720] WR:Wrote 0x23001d4 to out_ff:0x12460110
[3720] WR:Wrote 0x300 to out_ff:0x12460114
[3730] idx:6, rem:1, num:1, mode:0
[3730] emergency_shutdown_by_lowlow_battery: continue press power key!
[3730] LED ON+
[3740] LED ON-
[4240] LED OFF+
[4240] LED OFF-
[4750] LED ON+
[4750] LED ON-
[5250] LED OFF+
[5250] LED OFF-
[5760] LED ON+
[5760] LED ON-
[6260] LED OFF+
[6260] LED OFF-
[6770] LED ON+
[6770] LED ON-
[7270] LED OFF+
[7270] LED OFF-
[7780] LED ON+
[7780] LED ON-
[8290] LED OFF+
[8290] LED OFF-
[8790] Shutdo